1. Field of the Invention
This invention relates to earth boring and mining--especially to methods and apparatus for assembling underground a stem to the bit body of a raise boring head.
2. Background Information
Raise boring is a method of drilling a shaft between a tunnel and the surface of the earth or between two tunnels. A raise boring bit or head is employed with a stem attached to a drill pipe in a pilot hole. A drill rig at the surface pulls and rotates the pipe and stem to urge rotatable cutters upwardly into the earth. The cuttings fall into the shaft and are removed by rail car or other means.
The stem, raise boring head and cutters are often lowered separately into the tunnel, where they are then assembled in preparation for drilling. Some successful raise boring bits have stems and heads assembled by interference fit or other techniques using close fits between the interconnecting regions of the stems and heads. Care must be exercised to minimize damage to these close fitting, interconnecting regions prior to and during assembly. Damage can result from misalignment of the stem and head during assembly. The stem often has a hardfaced region that must pass through the head during assembly. Inaccurate alignment may damage the accurately finished region.
U.S. Pat. No. 4,069,878 discloses a raise boring bit that allows easy and effective installation and removal of a stem and head, assembled by regions of interference fit. Another method using untapered but close fitting regions is described in U.S. Pat. No. 4,228,863.
The most widely used method for installation of a stem underground is to attach a chain hoist to roof bolts in the tunnel roof. An effort is made to align the chain hoist with the centerline of the bit body. This procedure is inadequate when there is a tight fit between the wear pads and the bit body I.D. As the size of the pilot hole is increased to allow for larger diameter drill pipe, the clearance between the wear pads and existing bit body bores is decreased, and damage free installation becomes more difficult.
The slings in this prior art method can allow the stem to change position, both vertically and horizontally, and damage free assembly is tedious. A cardboard strip, temporarily placed over the hardfacing pads, has been used to minimize damage to the closely finished surfaces. But the cardboard strips have disadvantages in that they are often too thick and can be swept aside when the stem is being inserted in the bit body.